1. FIELD OF THE INVENTION
The present invention relates to a novel master batch of hardener for curable one-package epoxy resin systems. More particularly, the invention is directed to a latent master batch of hardener, mixtures of the hardener with epoxy resins being superior in storage stability, mixable with a great easiness, less affected by shear stress exerted thereupon during mixing, and producing hardened products of superior qualities, and a hardener composition thereof.
2. DESCRIPTION OF THE PRIOR ART
Epoxy resins are in extensive use for paints, electrical insulating materials, adhesives, etc., because hardened products of the resin have excellent performance characteristics. Curable epoxy resin systems generally used today are of the so-called two-package type composition, in which an epoxy resin and a hardener are mixed together just before use. Although curable at room temperature, the two package compositions type are bothersome to store and handle, since the epoxy resin and the hardener need to be stored separately and weighed and mixed before use as required. In addition, the limited pot life of the composition makes it impossible to mix both of the components in large amounts and hence the frequency of mixing them increases and the operation efficiency is inevitably lowered.
Some one-package type compositions have been proposed for the purpose of solving the problems associated with the two-package type ones. For example, there have been proposed compositions of epoxy resins with latent hardeners such as dicyandiamide, BF.sub.3 -amine complex, modified imidazole compounds (U.S. Pat. No. 4,066,625) and amine salts. Of these hardeners, dicyandiamide has long been known. When it is used alone to cure epoxy resins, curing temperatures of at least 170.degree. C. are necessary. The compositions thereof, however, when stored at room temperature, have shelf lives (pot lives) of at least 6 months. To decrease the curing temperature to the order of 130.degree. to 150.degree. C., there has been proposed the use of an accelerator such as a teriary amine, imidazole compound, or dimethylurea. The accelerator, however, shortens the pot life and thus fails to make the most of the latency of dicyandiamide.
Other approaches to one-package epoxy resin-hardener systems include; mixing an epoxy resin with an amine type hardener, followed immediately by freezing the mixture to stop their reaction; microencapsulating an amine type hardener; and adsorbing a hardener on a molecular sieve. However, the frozen type, microcapsule type, and molecular sieve type, at present, are inferior in performance characteristics, particularly in properties of the hardened products, and hence are scarcely put to practical applications.
Japanese Patent Publication No. 17654/68 describes a one-package epoxy resin system using a microencapsulated hardener. However, these microcapsules must be mechanically broken in order to cure the epoxy resin. Accordingly, this type of system does not meet the present requirement for epoxy resins to be cured without applying pressure. Japanese Patent Application Laid-Open No. 83023/83 discloses an approach comprising blocking functional groups present at the particle surface of a hardener with methyl iodide, acetic acid or the like, but simple blocking of functional groups at the surface cannot provide characteristics necessary for a one-package type composition, particularly long-term storage stability thereof.
Japanese Patent Publication No. 55970/83 and Japanese Patent Application Laid-Open Nos. 27914/84 and 59720/84, and EP-A-0193068 disclose that good hardener for one-package epoxy resin compositions can be obtained by combining powdery amine compounds with iso-cyanate compounds. Japanese Patent Application Laid-Open No. 59720/84 describes that the one-package characteristics can be achieved conceivably by deactivation due to the isocyanate compound being present at the surface of the powdery amine compound. As stated above, however, properties necessary for the one-package type composition, particularly good storage stability, cannot be maintained by simple blocking of functional groups at the surface. For the practical use of a one-package type composition uniformity of the composition is important; that is, a powdery hardener should be dispersed uniformly in the epoxy resin by means of a roller mill or other equipment. However, the methods disclosed in these four patent applications have a drawback in that inert surface layers once formed are ruptured by shear stresses exerted during dispersion at room temperature and this worsens the storage stability of the objective composition.
As stated above, one-package type epoxy resin compositions according to the prior art have drawbacks, each failing to exhibit the advantage of storage stability.